Conventional rivets do not provide a satisfactory assembly of composite materials. Frequently, the radial expansion of the rivet shank and the pressures exerted underneath the post-shaped head may cause de-lamination of the material and unglueing of fibers and binder, so that the assembly poorly resists fatigue, and there is a danger that the rivet will come loose prematurely.
U.S. Pat. No. 4,221,041 describes a semi-tubular rivet which is supposed to overcome these drawbacks. However, testing has shown that such a rivet still undergoes a large expansion beneath the post-shaped head, and overall a fairly substantial swelling along the other segments of the shank, whereby the fatigue strength of the assemblies so made as a rule are not satisfactory, particularly in aeronautical applications.
Furthermore, the set-up conditions (applied pressure) affect considerably the quality of the assembly and these rivets must be set at a very precise riveting pressure. However, in practice such an operation is very delicate and difficult to achieve. Finally, a known rivet of a given size is suitable for assembling parts of precise thickness with only narrow tolerance, and under these conditions, the industrial application of these rivets raises substantial practical difficulties.
Accordingly, there is only a poor solution presently available for the problem of riveting soft or fragile materials.
The present invention, therefore, provides an improved rivet allowing the assembly of soft or fragile parts, particularly parts of composite materials, without significant degradation of the material in the area near the rivet, in order that the composite material may retain its original strength.
To that end, the primary object of the present invention is to provide a rivet which can be set without undergoing significant expansion or swelling, whether underneath the post-shaped head or along other parts of the shank.
Another object of the invention is to facilitate the rivet set-up conditions by permitting a wide range of riveting or assembly pressures.
A further object of the invention is to significantly increase the range of thicknesses which can be assembled by the same type of rivet, in order to facilitate the industrial use of these rivets.
Still another object of the invention is to provide an improved assembly process for composite materials.
These and other objects and advantages of this invention will become apparent upon further consideration of the invention, on the basis of the following specification and claims, together with the accompanying drawings.